High power electron beam phase shifter



Sept. 3, 1968 D. WINSOR HIGH POWER ELECTRON BEAM PHASE SHIFTER 2 Sheets-Sheet 2 Filed March 31, 1965 hzwmmno E SNTZSEOZ NORMALIZED DISTANCE --2 INVENTOR oamuo- L. W/MSOR ATTORNEY United States Patent 3,400,296 HIGH POWER ELECTRON BEAM PHASE SHIFTER Donald L. Winsor, Wakefield, Mass., assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed Mar. 31, 1965, Ser. No. 444,187 14 Claims. (Cl. SIS-3.5)

ABSTRACT OF THE DISCLOSURE An electronic phase shifter device for controlling the propagating characteristics of electromagnetic wave energy by means of controlling the power level of an electron beam. The control of the reactive current component of the electron beam which is of incidental consideration in prior art traveling wave interaction type amplifier or oscillator devices is determined to be the primary phase shifting control means. The device is operated at a level where substantially no amplification of propagated microwave energy results and very low electron beam power values will control relatively high power electromagnetic wave energy.

The present invention relates generally to microwave electron devices and in particular is directed to such devices for electronically controlling the phase characteristics of electromagnetic wave energy in a predetermined manner with a high degree of reproducibility and accuracy.

Improvements in radar design have resulted in present day phased array systems in which the antenna remains stationary while the radar beam is transmitted in a scan pattern at very rapid rates. In multi-dimensional phased array antennas a large number of radiating elements are provided at equidistant points. With the proven feasibility of extremely large electronically-scanned phased array antennas, the need for suit-able devices and circuitry has arisen for accurately shifting the phase of energy directed to each individual antenna radiating element. The present day designer is faced with extremely stringent requirements due to the necessity for maintaining the large number of antenna radiators in a fixed interconnected relationship to coherently radiate the desired wave front. Further state-of-the-art improvements have led to the employment of electronically controlled phase shifters which are capable of providing beam scanning rates which are considerably faster than those attainable with mechanical phase shifting components. Thus, traveling wave tube amplifier chains to feed the antenna elements have been utilized together with semiconductor diodes and/ or ferrite devices to result in rather involved hybrid plumbing arrangements in the generation of the phased array wave front.

The traveling wave amplifier type of electron tube commonly incorporates a slow Wave periodic structure, such as a helix, of a fixed predetermined physical length. An electron beam is transmitted axially through the periodic slow wave structure at a velocity synchronous with the progression or phase velocity of the microwave energy propagated adjacent thereto. Containment of the electron beam is provided for by surrounding electrostatic or magnetic fields. Such tubes are further provided with an attenuator section in the slow wave structure to effect reduction in the velocity of the electromagnetic wave energy to achieve the state of synchronization wtih the electron beam. The electrical phase length of the traveling wave amplifier tube may be varied by varying the accelerating voltage of the electron beam. Means to control the velocity of the beam, therefore, are commonly provided to control the phase length of the tube. In the conventional traveling wave amplifier the order of magnitude of the electron beam power is considerably greater than the microwave signal output to result in effciency ratings of only approximately 20 percent in the amplification of microwave signal energy. As a result, to achieve an output RF signal of one Watt, approximately twenty watts of beam power is required. It will be realized that such a ratio of output signals to electron beam power places tremendous on-site power requirements on the over-all system when considering phased array antennas employing hundreds or even thousands of ganged traveling wave amplifier tubes. To design even longer distance phased array systems would require the construction of huge power generation facilities together with the antenna systems which leads to a prohibitive cost situation.

In the phased array type of radar antenna system, a number of desirable characteristics for electronically controlled phase shifting devices may be noted. First and foremost, the phase setting must be nearly independent of frequency, temperature and power level. Next, the insertion loss for each phase shifting component must be maintained at a reasonably low level. Further, power handling capabilities must permit radiation of total transmitter power without electrical breakdown or phase distortion. Finally, the electrical inertia should be low to permit the phase shifting function to occur within the required beam shift interval with the available driving power of the system. Numerous other requirements such as weight, size, and cost are also of vital consideration for the designer of the over-all phased array system.

The electronic phase shifter envisaged in the present invention provides for the variation of the phase of the microwave electromagnetic energy transmitted through the device by means of control of an electron beam having a. power substantially less than the microwave signals to be phase controlled. Microwave energy is propagated through the interior of an evacuated envelope housing a periodically loaded slow wave structure having a configuration similar to a helix, cross-wound helix or like structures. At all times, the propagation characteristics of the electron beam are maintained in a velocity synchronous relationship with the wave propagating upon the slow Wave structure and at a sufiiciently low energy level to avoid growing or decaying wave interaction between the electron beam and the energy on the slow wave structure. Stated in its simplest form, the invention contemplates a device having means for varying phase lengths by control of the electron beam current in such a manner that as the current is increased the phase length of the device also increases linearly.

It has been observed in studies of electron beam characteristics that in the presence of the E-field component of electromagnetic microwave energy the beam is sufficiently perturbed to result in a plurality of electron packets with minimal net energy exchange. The electron packets display a microwave current having a fundamental frequency component at the frequency of the microwave energy propagating along the slow wave structure. This microwave current is considered to be reactive in nature and has been determined to be a function of electron beam current. Consequently, variations in the magnitude of the reactive current component varies the propagation constants of the microwave energy along the slow wave structure. The invention therefore discloses an electronic phase shifting device wherein no amplification of the microwave energy results and the phase control power factor is considerably reduced below the microwave energy power level. Illustrative, in an embodiment of the present invention phase control of the propagation characteristics of twenty watts of microwave energy would require only one watt of beam power. Such a reduction in the power requirements readily lends itself to the larger phased array systems and permits the utilization in such systems of extremely high power microwave generators such as th: magnetron amplifier described in the article A New Microwave Tube Device by William C. Brown, Proceedings of the IRE, vol. 45, p. 1209, September 1957, capable of efficiencies as high as 70 percent. An equation is also disclosed to accurately design individual electron tubes for each desired phase shift value. Actual embodiments in various diverse frequency levels have verified the concept disclosed herein as well as the empirically determined constants in the aforesaid equation.

The present invention has for its primary object the provision of an electronic phase shifter capable of providing predetermined phase characteristics for microwave electromagnetic energy propagation.

Another object of the present invention is the provision of a new and improved electronic phase shifter for use in phased array antenna systems to provide for rapid scanning of equidistant isotropic radiating elements with a minimum of power required for on-the-site control of the phase characteristics of individual elements.

Other objects, features and advantages will become apparent after consideration of the following detailed specification together with the accompanying drawings, in which:

FIG. 1 is a schematic view useful in explaining the principle of operation of the invention;

FIG. 2 is a graph plotting normalized microwave RF current over a normalized distance of the microwave propagating structure;

FIG. 3 is a graph plotting phase shift per unit of beam wavelength versus the ratio of beam power to microwave RF power;

FIG. 4 is a graph plotting phase shift in degrees versus collector current for an illustrative phase shifting device operative in the L-band frequency band; and

FIG. 5 is an illustrative embodiment of a high perveance hollow electron beam generating means for incorporation in the phase shifting device.

Referring to FIG. 1, a schematic view of the embodiment of the invention is shown and designated 2. A periodic slow wave microwave energy propagating structure 4 is aligned coaxially within envelope 6. The slow wave structure has been shown for illustrative purposes as a unifilar helix; however, such structures as a bifilar helix, ring-bar line or similar arrays may be employed depending upon the design parameter of the over-all tube in accordance with the equations to be hereinafter discussed. A glass envelope 6 is preferred although in certain applications it may be desirable due to long length to provide a metallic envelope. In such tube structures the internal slow wave structure and electrode leads will be insulated from the envelope by means of ceramic spacers. Microwave energy is coupled into and out of the respective ends of the slow wave structure by input transition means 8 and output transition means 10. Electron beam emitter means 12 are disposed at the end of the tube envelope adjacent the input means to propel the stream of electrons along the tube axis and axially within the slow wave structure. Adjacent to the opposing end of the envelope a collector electrode 14 is shown to provide a terminal for the electron beam indicated by the dotted lines 16. Surrounding the overall tube envelope is a magnetic member 18 to provide a longitudinal magnetic field parallel to the path of the electron stream. The electron emitter 12 comprises an emissive cathode 2t) and heater coil 22 with suitable leads extending through the tube envelope for connection to appropriate biasing voltage supplies. Beam focusing anode member 24 is provided in the intermediate region between the input end of the helical structure and the electron emitter and is provided with a sufficiently high positive voltage to accelerate the emitted electrons along the desired path of travel. A grid member 26 is disposed adjacent to the emissive cathode and is suitably biased by connection means extending through the end of the envelope to a DC variable supply to provide for control of the electron beam current independently of the applied voltage to the accelerating anode 24.

An investigation of the state of electron beam trajectories in the presence of large microwave electric fields has revealed that electrons within the beam become trapped so that electron packets ride the wave with a minimal net energy exchange. The electron packets when analyzed reveal a microwave current I having a fundametal frequency component at the frequency of the microwave energy propagated upon the adjacent slow wave structure. Referring to FIG. 2, results of rigorous mathematical computations are plotted to further substantiate this observation. J. R. Pierce, in the text Traveling Wave Tubes, D. Van Nostrand Co., New York (1950), in his theoretical analysis of traveling wave tube phenomena has derived certain equations to show the effect upon electrons in the electron beam when acted upon by the AJC. field and displacement of these electrons in the axial direction only is considered. Assuming a device consisting of a transmission line coupled to a longitudinal electron beam, the transmission line equations on page 10 of the Pierce reference are as follows:

where and V are the current and the voltage in the transmission line, B and X are the shunt susceptance and series reactance per unit length and J is the impressed current per unit length.

Since Pierces work is relevant only with a very small microwave energy signal at the input of the helical structure, a modification of the transmission line equations is necessary in order to calculate transmission line equations for the device envisaged in the present invention. In particular, the present invention relates to high power microwave energy signals presented at the input of the slow wave transmission line wherein the R.F. power is great enough to result in an electric field which is almost the same with or without the adjacent electron beam. The trajectories of the electrons introduced to the high power microwave field are calculated from Newtons laws by the equations:

The normalized microwave R.F. current I AC. was approximated from the equation and is plotted as the dotted line 30 in FIG. 2 as computed over the distance Z of the helix structure. The real and imaginary current plots are shown by the dotted and solid lines 30 and 32 with the dotted line 30 indicating the imaginary components and the solid line 32 the real. The respective curved plots indicate that the sinusoidal normalized R.F. current displays a reactive component.

In accordance with the teachings of the present invention it is this microwave current in the electron beam which is considered as the perturbing element in variation of the propagation characteristics of the electronic phase shifter. The behavior of a velocity-synchronous electron beam directed in the z direction along the helix axis in the region of a high microwave electric field may be mathematically described as follows: ocE. L,

rt (6) Using the definition of interaction impedance according to Pierce in the aforementioned reference when A9=Phase shift in radians 1 =Electron beam current in amperes L=Length of interaction in cm. 1I=Frequency in gc./s.

K=Beam interaction impedance in ohms V =Beam voltage in volts P =Microwave power in watts =Empirically determined constant Employing the referenced Pierces gain parameter, p. 253,

where K=helix impedance, l beam current and V beam voltage and a value of 20 for the constant as determined by numerous experimental models at UHF and microwave frequencies the basic design equation for the phase shifter of the present invention is determined to be as follows:

where P is the value of beam power in watts and P is the value of the microwave power in watts and C is the Pierce gain parameter.

As a result of the precepts of the invention, phase shift has now become a direct function of electron beam current and is inversely proportional to the square root of the microwave power propagated through the device. FIG. 3 plots the above Equation 9 for different values of the gain parameter (C) to illustrate the operation of the invention. It will be noted that the curves 34, 36, 38 provide a means for fabrication of a phase shift device to meet individual requirements for application in phased array antenna systems.

Numerous embodiments of the high power electron beam phase shifter utilizing the teachings of the invention were fabricated for operation at various frequency bands including UI-IF, S and L bands. 'In all of these models the linear relationship between phase shift and beam current was successfully demonstrated. Each of the electron gun assemblies employed was provided with a current control electrode to permit variation of the electron beam current independently of the anode voltage. At S-band the electron beam emitter was capable of generating a 1.5 peak ampere solid beam at 9.5 kilovolts. The phase shift was found to be a maximum at 7.25 kilovolts for a frequency of 2.8 Gc and a collector current of .5 amperes. The microwave RF peak power transmitted through the adjacent slow wave helix structure was rated at 50 kilowatts peak at a 0.001 duty cycle. The over-all length of the helix was approximately 9 inches long. It is of interest to note that the maximum phase shift occurred at a value of 7.25 kilovolts for the electron beam voltage for a helix slow wave structure having a measured small signal gain synchronous voltage value of 8.0 kilovolts. This confims the theoretical considerations wherein it was predicted that under conditions of velocity synchronism the electrons in the beam when exposed to large microwave E-fields combine into electron packets which acquire the sinusoidal movement of the RF energy without any measurable net energy exchange between the beam and the RF energy.

At L-band illustrative embodiments of the invention were provided with an electron emitter capable of generating a high perveance beam. Since the perveance of the solid electron beam is limited to approximately 2 10 it is advantageous to use a hollow electron beam capable of perveances as high as 5X10' An exemplary structure capable of generating up to 5 amperes current at 10 kilovolts is illustrated in FIG. 5.

A hollow beam source similar to the so-called magnetron injection gun comprises a cathode member 42 with an internal heater coil 44. A positively biased modulating anode 46 of a modified frusto-conical configuration provides for the shaping of the electron beam as the electrons are emitted from the cathode. This anode member permits adjustment of the electron beam current to achieve desired phase shift. A longitudinal electrostatic or magnetic field indicated schematically by numeral 48 provides for containment of the emitted hollow beam along the axis of the envelope 50. The path of the electron trajectory as the beam enters the slow wave structure is shown by dotted lines 52. The over-all diameter of the illustrative beam capable of 5 amperes at 10 kilovolts was approximately 0.600 inch.

In FIG. 4 the results of an L-band.embodiment at a frequency of 1347 megacycles are plotted to show the phase shift in degrees as a function of the collector current in amperes. Curve 40 clearly indicates the linear relationship of beam current to phase shift since in the embodiment of the invention with minimal energy exchange between the electron beam and the slow wave structure the collector current is essentially a direct measurement of the electron beam current. In this embodiment utilizing the cathode capable of a high preveance beam 60 kilowatts of microwave RF energy was successfully propagated at a substantially invariant level while the propagating characteristics were accurately controlled by means of adjustment of the electron beam current. In this structure the slow wave helix line was approximately 20 inches in length which further indicates the easy adaptability of the invention to present day periodically loaded waveguide propagating structures having lengths in this region. In all the structures tested noise did not present any problem and the utilization of the embodiments utilizing the equations seems to be only limited by the state of the art of electron gun and slow wave structure technology. In addition, gain is not a relevant characteristic since the present device is not an amplifier of microwave or ultra high frequency energy but only a high power electronic phase shifter. It is rather a device functioning as 'a linear accelerator to effect changes in propagating character istics.

A new and novel means for electronic control of phase shift has been disclosed having power requirements substantially lower than the microwave power being propogated. The device functions as a transparent element of electronically variable phase length through control of the current in an electron beam. The wave propagating structures are designed for maximum RF interacting field characteristics with minimal net energy exchange between the beam and Wave structure. The wave propagating structures are made free of loss to facilitate minimum microwave dissipation through the device. The device further lends itself to the so-c'alled depressed collector electrode configurations which teaches variation of potential and shape of this electrode with respect to the source of the electron beam source for maximum utilization of the beam in the region adjacent the RF propagating structure. The equation provided herein permits construction of devices for any frequency bands with full coverage of phase shift range.

Several emitters as well as propagating structures have been illustrated or specified herein by way of explanation only and are not intended as limitations. Many modifications and alterations of the present invention may be evident to skilled artisans and it is intended that such alternative embodiments be included within the purview of the spirit and breadth of the interpretation accorded the appended claims.

What is claimed is:

1. An electronic phase shifter comprising:

a slow wave electromagnetic energy propagating structure;

means for emitting an electron beam adjacent to said slow wave structure;

said electron beam being in velocity synchronous relationship with electromagnetic energy propagated upon said slow wave structure at a sufficiently low power level to minimize interaction between the electron beam and the electromagnetic energy;

and means for varying the electron beam current to thereby vary the effective electrical length of said slow wave structure and alter the phase velocity characteristics of the electromagnetic energy traversing said slow wave structure.

2. An electronic phase shifter comprising:

a slow wave electromagnetic energy propagating structure;

means -for emitting an electron beam 'along an extended path adjacent to said slow wave structure;

said slow wave structure having a physical length and electrical characteristics to establish a velocity synchronous relationship between the electron beam and said structure at a sufficiently low power level to minimize interaction between the electrical fields of said electromagnetic energy and the electron beam;

and means for varying the electron beam current to vary the effective electrical length of the slow wave structure linearly with respect to linear variations of the beam current.

3. An electronic phase shifter comprising:

a slow wave electromagnetic energy propagating structure of a predetermined electrical length;

means for emitting an electron beam along an extended path adjacent to said slow wave structure;

means for propagating an electromagnetic wave 'along said slow wave structure to induce a reactive micro- Wave current in close proximity to said electron beam;

and means for varying the electron beam current to thereby effectively vary the magnitude of said reactive microwave current and alter the phase velocity of energy propagated on the slow wave structure.

4. An electronic phase shifter comprising:

a slow wave electromagnetic propagating structure having physical and electrical parameters designed for progression with predetermined phase velocity of electromagnetic wave energy;

means for generation of an electron beam;

means for propelling said beam at a velocity synchronous with the progression and phase velocity of the electromagnetic energy along a path adjacent to said slow wave structure;

means for coupling said energy to said slow wave structure;

means for controlling the current in said electron beam;

means for varying the electron beam current independently of the propelling means to linearly vary the effective electrical length of said slow wave structure and the phase velocity characteristics of the energy propagated on said slow wave structure;

and output coupling means for removing said electromagnetic wave having altered phase velocity characteristics and essentially the same order of magnitude of power as the energy introduced initially to the slow wave structure.

5. An electronic phase shifter comprising:

a slow wave structure having selected physical and electrical parameters designed for progression with predetermined phase velocity of microwave electromagnetic energy;

means for generating a high perveance electron beam and means for directing said beam along an extended path adjacent to said slow wave structure;

means for propagating an electromagnetic wave on said slow wave structure;

means for linearly varying the electron beam current to linearly vary the phase velocity of the electromagnetic waves, all of said means being appropriately calculated from the equation:

where:

A6=phase shift in radians, QC =a constant 20, I -=electron beam current in amperes, L=length of interaction region in cm., v=frequency in gc./s., K=-beam interaction impedance in ohms, V zbeam voltage in volts and P =microwave power in watts;

and means for removing said electromagnetic energy having the desired phase velocity characteristics from said slow wave structure.

6. An electronic phase shifter comprising:

a slow wave structure having selected physical and electrical parameters designed for progression with predetermined phase velocity of microwave electromagnetic energy;

means for generating a high perveance electron beam and means for directing said beam along an extended path adjacent to said slow wave structure;

means for propagating an electromagnetic wave on said slow wave structure;

and means for linearly varying the electron beam current to linearly vary the phase velocity of the electromagnetic wave, all of said means being appropriately calculated from the equation:

where:

where K=the helix impedance, I =the beam current and V '=the beam voltage, P -beam power in watts and P =microwave power in watts.

7. An electronic phase shifter according to claim 6 wherein said means for varying the electron beam current comprise a control electrode disposed in close proximity to said electron beam generating means with a variable DC power supply connected to said control electrode.

8. An electronic phase shifter according to claim 6 wherein said means for generating an electron beam comprise a hollow beam electron emitter having an electron emitter and a surrounding positively biased modulating anode, and said means for varying the electron beam current comprise a variable power supply connected to said modulating anode member.

9. An electronic phase shifter comprising:

an envelope;

a slow wave structure having selected physical and elec trical parameter design for progression with predetermined phase velocity of electromagnetic energy positioned along the envelope axis;

an electron source adapted to emit an electron beam;

means for propelling said beam axially along an eX- tended path defined by said slow wave structure in velocity synchronous relationship with the electromagnetic energy propagated on said slow wave structure at a power level to result in minimal net energy exchange;

a control electrode disposed at an intermediate point between said electron source and propelling means;

a DC variable power supply connected to said control electrode to vary the electron beam current to thereby result in alteration of the phase velocity characteristics of the energy propagated on said slow wave structure Without amplification;

and input means for coupling electromagnetic energy to said slow wave structure and output coupling means for removing said energy at essentially the same order of magnitude of power initially introduced to said slow wave structure.

10. An electronic phase shifter according to claim 9 wherein said electron source is adapted to emit a hollow beam and said control electrode surrounds said electron source.

11. An electronic phase shifter according to claim 9 wherein said slow wave structure comprises a helical delay line means without attenuation means.

12. An electronic phase shifter according to claim 10 wherein said hollow beam source has a perveance value about 2 X10 13. In a phased array antenna system the combination:

a single high power electromagnetic energy source transmitter;

a plurality of electronic nonamplifying phase shifters coupled to said transmitter;

each of said phase shifters comprising a slow wave structure having selected physical and electrical parameters designed for progression with predetermined phase velocity of said electromagnetic energy;

input and output means for propagating said energy on said slow wave structure; means for generating a high perveance electron beam; means for propelling said beam at a velocity syn- 5 chronous with the progression and phase velocity of the electromagnetic energy along a path adjacent to said slow wave structure; means for controlling the electron beam current independently of said propelling means to linearly adjust the effective electrical length of each slow wave structure, all of the parameters of said means being calculated for individal phase shifters by the equa tion:

References Cited UNITED STATES PATENTS 2,697,169 12/1954 Emslie 328254 X 2,776,374 1/1957 Iskenderian SIS-3.5 X 2,806,177 9/1957 Haelf 315-35 X 2,930,932 3/1960 Geiger 313157 X 3,007,077 10/1961 Geiger 3153.5 3,028,597 4/1962 Cicchetti et al. 3153.5 X 3,153,742 10/1964 Kluver 315-35 X ELI LIEBERMAN, Primary Examiner. S. CHATMON, 1a., Assistant Examiner. 

